Monitoring Warfarin with the INR
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Since the Prothrombin time is sensitive to multiple factor deficiencies, it is effective in measuring the effect of vitamin K antagonists. Modern management utilizes an expression of the prothrombin time called the INR which relates individual prothrombin times performed at various sites using various instrument and various reagent sources to the Word Health Organization standard reagent. This allows for uniformity of expression of the prothrombin time to allow for uniform interpretation and common communication with regard to oral anticoagulant therapy. Each reagent source and instrument combination is compared to the WHO standard to determine the International Sensitivity Index (ISI). The ISI is published with the reagent insert and varies by lot number. The laboratory uses the ISI to calculate the INR in the following formula.

The INR is only relevant for patients on a stable regimen of oral anticoagulants (warfarin /coumadin).

The bottom line?
The INR allows for comparison of data from various labs, even from different countries. This allows for monitoring of patients who travel frequently or who use different labs within a region. It also allows for better communication between physicians in reaching optimum levels of anticoagulation with minimal side effects.
The recommended therapeutic levels for standard oral anticoagulant therapy will yield an INR of 2.0-3.0.
For high-dose therapy the INR target is 2.5-3.5.


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Page updated: 9/24/04